1. Field of the Invention
This invention relates to improvements in a dual-mode drum brake which functions as the service brake in the form of a leading/trailing type brake during running of an automotive vehicle, and as the parking brake in the form of a duo-servo type brake.
2. Discussion of the Related Art
Generally, a leading/trailing type drum brake includes (a) a backing plate, (b) a wheel cylinder having a pair of pistons, (c) an anchor, and (d) a first brake shoe and a second brake shoe. The leading/trailing type drum brake is widely used as the service brake on an automotive vehicle, for decelerating the vehicle during running thereof. The backing plate is a generally circular disc which is a stationary member fixed on the vehicle such that the disc is opposed to an opening of a brake drum which rotates with a wheel of the vehicle. The wheel cylinder is a generally cylindrical member which is open at its opposite axial ends, and is disposed on a radially outer portion of the backing plate such that the axial direction of the wheel cylinder is substantially parallel to the circumferential direction of the backing plate. The two pistons are axially outwardly movable from the opposite open axial ends. The anchor is fixed to a circumferential portion of the backing plate that is diametrically opposite to the portion at which the wheel cylinder is disposed. The first and second brake shoes are supported by the backing plate movably in a plane of the backing plate, such that the opposite ends of the two brake shoes are opposed to the opposite ends of the wheel cylinder and the anchor. In operation, the two pistons of the wheel cylinder are moved in the opposite directions away from each other, so that each of the brake shoes is forced against the brake drum rotating with the vehicle wheel, with one of its opposite ends held in abutting engagement with the corresponding end of the anchor, whereby the wheel is braked.
As one type of the leading/trailing drum brake constructed as described above, there has been proposed a dual-mode drum brake wherein the parking brake for braking the vehicle during parking thereof acts as a duo-servo brake which is capable of providing a comparatively large braking force by a duo-servo mechanical brake action.
An example of the dual-mode drum brake is disclosed in U.S. Pat. No. 5,070,968 to A.C. Evans. This drum brake, which will be referred to as "first prior art drum brake", includes (a) an intermediate lever which is disposed substantially in parallel with the first brake shoe and connected to an intermediate portion of the first brake shoe such that the first brake shoe is pivotable about a first pivot axis perpendicular to the plane of the backing plate, (b) a pair of struts which are disposed substantially in parallel with each other and associated with the opposite end portions of the intermediate lever and the second brake shoe, (c) a parking lever which is connected to one end portion of one of the pair of struts that is located on the side of the anchor, such that the parking lever is pivotable about a second pivot axis substantially parallel to the plane of the backing plate, so that a pivotal movement of the parking lever about the second pivot axis causes the anchor side strut and the intermediate lever to be moved away from each other, for thereby applying a parking brake to the vehicle wheel.
Another example of the dual-mode drum brake is disclosed in U.S. Pat. No. 5,275,260 to A.C. Evans et al. This drum brake, which will be referred to as "second prior art drum brake", includes a parking lever which is disposed substantially in parallel with the second brake shoe and which is connected to the anchor side end portion of the second brake shoe pivotally about a second pivot axis perpendicular to the plane of the backing plate, so that a pivotal movement of the parking lever about the second pivot axis causes the anchor side strut and the second brake shoe to be moved away from each other, for thereby applying a parking brake to the wheel.
The first prior art drum brake is disadvantageous in that it is difficult to design the parking lever with a sufficient length. This difficulty arises from the orientation of the parking lever so as to extend in the direction perpendicular to the plane of the backing plate. This orientation requires the length of the parking lever to be accommodated in the axial dimension of the brake drum, and tends to limit the length of the parking lever. The limitation in the length of the parking lever in the first prior art drum brake makes it difficult to meet a general parking brake requirement that the pivoting angle of the parking lever be minimized while at the same time the operating stroke of the parking lever at its point of connection to a parking brake operating cable be maximized.
The parking brake operating cable is accommodated in an outer tube, and is fixed at one end thereof to the parking lever and at the other end to a parking brake operating member through an equalizer, an intermediate lever, etc. The outer tube is fixed at one end thereof to the backing plate and at the other end to the frame of the automotive vehicle. Upon operation of the parking brake operating member, the parking brake operating cable is moved relative to the outer tube, and an operating force is transmitted from the operating member to the parking lever of the drum brake assembly. Since the cable and the outer tube are curved and there exists a clearance between the diameter of the cable and the inside diameter of the outer tube, the other end of the outer tube remote from the backing plate and the corresponding portion of the cable can be moved relative to each other by a given distance even when the end of the outer tube fixed to the backing plate and the corresponding portion of the cable are not moved relative to each other. Namely, there exists a given amount of play between the cable and the outer tube. This play causes an undesirable increase of the required operating stroke of the parking brake operating member. Further, the operating stroke is unnecessarily increased by elongation of the cable and contraction of the outer tube. The amounts of these play, elongation and contraction largely depend on the specific construction of the parking brake. It is desirable to reduce the ratio of the ineffective portion of the overall operating stroke of the parking brake operating member, for increasing the ratio of the effective portion of the operating stroke which is used for moving the portion of the parking lever of the drum brake assembly at which the cable is fixed. This ratio of the effective portion of the operating stroke of the parking brake operating member can be increased by increasing the operating stroke of the parking lever at its cable fixing portion, that is, by increasing the length of the parking lever. An increase of the length of the parking lever also results in a decrease in the required pivoting angle of the parking lever.
In the first prior art drum brake, however, the length of the parking lever is limited since the length should be accommodated within the axial dimension of the brake drum as discussed above. Accordingly, the first prior art drum brake cannot meet the above-indicated requirement.
The comparatively short parking lever has another drawback, that is, a relatively low freedom of design relating to the lever ratio, more specifically a ratio of a distance between the pivoting axis of the parking lever and the cable fixing portion of the parking lever (to which the parking brake operating force is applied), to a distance between the pivoting axis of the parking lever and a point at which the force acts on the brake shoe through the intermediate lever. Thus, the first prior art drum brake suffers from difficulty to increase the boosting ratio. Accordingly, the tensile and compressive forces applied to the cable and the outer tube tend to be comparatively large, requiring the cable and outer tube to have an accordingly high strength, undesirably increasing the weight and cost of manufacture of the cable and outer tube.
In the second prior art drum brake, on the other hand, the parking lever extends substantially in parallel with the second brake shoe, and is pivotally connected to the appropriate end portion of the second brake shoe such that the parking lever is pivotable about the axis perpendicular to the plane of the backing plate. This arrangement permits the parking lever to have a comparatively large length, which makes it easier to increase the operating stroke of the parking lever at its cable fixing portion and the boosting ratio while reducing the required pivoting angle of the parking lever. However, since the cable fixing portion of the parking lever is located on the side of the wheel cylinder, the cable should extend through a space between the parking lever and the wheel cylinder, leading to relatively difficult positioning of the cable through the portion of the vehicle frame near the drum brake assembly. Described in detail, the drum brake assembly is usually oriented such that the wheel cylinder is located right above the axis of the brake drum. In the portion of the vehicle frame in the vicinity of the wheel brake cylinder, there are disposed various components such as a coil spring and other suspension parts, and some parts of the vehicle body. For facilitating the installation and positioning of the cable, the parking lever is desirably supported at a portion thereof on the side of the wheel cylinder, while the cable fixing portion is desirably located on the side of the anchor. In the second prior art drum brake, however, the cable fixing portion is located on the side of the wheel cylinder.
The wheel cylinder may be offset from the upper portion of the backing plate in the circumferential direction of the backing plate. In this case, too, it is difficult to position the parking brake operating cable if the cable fixing portion is located on the side of the wheel cylinder, because the cable should be installed so as to avoid an interference with a piping for applying a pressurized fluid to the wheel cylinder.